1. Field of the Invention
This invention relates to a method and a device for monitoring the transparency of a pane, especially a laminated pane.
2. Description of the Relate Art
The term laminated pane is to be understood as meaning, according to this invention, a pane composed of at least one rigid sheet of glass or plastic material and at least one layer or flexible film of plastic material. These laminated panes may be panes known as symmetrical panes when they comprise two rigid sheets, notably of glass, as outer sheets. The laminated panes may also be panes of the types known as asymmetrical when they contain one rigid sheet and one sheet of flexible plastic material as the outer sheets.
These laminated panes have advantageous properties, notably a high impact resistance; they find applications as a burglar-proof pane in building construction, for instance, or as a window pane in building construction, for instance, or as a window pane in transport vehicles (automobiles, aircraft, trains, etc.).
A laminated pane should possess a specific transparency, depending upon its application. In the case of an automobile pane, the light transmission should be greater than 70%, and this value must be maintained with time. Methods of monitoring the transparency therefore are obligatory.
In the case of laminated panes, the achieving and maintenance of the optical quality are linked to the quality of the assembling process. This assembling process can be broken down, generally, into two steps: preliminary assembling and final assembling.
Various techniques are used for carrying out the preliminary assembling. One of these techniques is that known as "calendaring". There, the various elements intended for making up a laminated pane, superimposed upon one another in the desired arrangement, are heated and are subjected to a pressure applied by the rollers of the calendar. The combined action of heating and pressure causes a bond between the various elements, which are thus joined to one another.
Another technique is one that uses the conjugate action of vacuum and heating to cause an adhesion between the various elements making up a laminated pane. These elements, superimposed in the desired arrangement, are introduced into a hermetically sealed pouch or bag. This pouch is heated and subjected to vacuum, thus causing adhesion between the various elements. The laminated assembly is then removed from the pouch.
The final assembling together is generally carried out in an autoclave, where the preassembled laminated assembly is subjected to the simultaneous action of temperature and pressure.
It is as a result of this final assembling that the pane acquires its final transparency.
In certain panes a tendency to increasing opacity with time has been observed, whereas these same panes possessed suitable transparency when they left the final assembling operation. Research has shown that the transparency of the pane and its evolution with time are a function of the conditions under which the preliminary assembling operation was carried out. There was therefore a requirement to monitor the preliminary assembly operation in order to limit the risks of later failure. This monitoring may be performed by verifying the parameters that are involved in the assembling operation, in particular in the case of assembly inside a pouch or bag. These parameters are, notably, temperature and pressure or vacuum. This control of parameters is, however, not sufficient for ensuring a good preliminary assembly, since other factors can come into play, such as a defect in the pouch, or a fold in the pouch leading to inadequate vacuum, etc.
Another solution is to monitor the transparency of the laminated pane as it leaves the preassembling operation.
In the case where a laminated pane of the conventional type is involved, composed of two glass sheets and one intermediate sheet or film of polyvinyl butyral (PVB), the preliminary assembling operation results in a partial transparency of the pane which, initially, is not transparent on account of the structured surface condition of the two faces of the PVB sheet, which must allow the flow of air between the sheets during the course of the assembling operation.
The transparency of the pane at this stage of manufacture is only partial, and frequently inhomogeneous. It is therefore difficult to monitor this transparency. It is possible for an individual person to monitor the preassembled pane visually, but this check is subjective and expensive in labor costs.